The typical backpack is a bag with a closure and two shoulder straps, which are slung over the shoulders so that the weight of the backpack is distributed on the shoulders and the backpack is carried on the upper back. Each shoulder strap is attached near the upper corner and lower corner of the bag, and each makes a rough loop, between the strap and the bag, through which one of the wearer's arms is stuck. The shoulder straps are typically made from padded material and nylon webbing, to make it more comfortable for the wearer's shoulder to bear the weight of the backpack.
Generally, using two shoulder straps is preferable to slinging the backpack over a single shoulder because the weight load is distributed between two shoulders, making it easier to carry heavy loads for longer periods. Single-shoulder-strap backpacks, such as the common “messenger-bag” style do not offer this, and other, advantages.
The major drawback of using two shoulder straps and carrying the backpack on the back is that the backpack can not be accessed while wearing. That is, accessing the contents of the backpack requires taking at least one shoulder strap off.